What is a Spectrum Analyzer: RF spectrum analyzer

Overview & tutorial about the basics of a spectrum analyser: what one is; how to use a spectrum analyser; and how one works.


Spectrum Analyzer Tutorial Includes:
What is a spectrum analyzer     Spectrum analyzer types and technologies     Superheterodyne / sweep spectrum analyzer     FFT spectrum analyzer     Realtime spectrum analyzer     USB spectrum analyzer     Spectrum analyzer tracking generator     Specifications     Spectrum analyzer operation     Noise figure measurements     Phase noise measurements     Pulsed signal spectrum analysis    


A spectrum analyzer is a very important item of test equipment for someone designing or repairing electronic equipment that uses radio frequency signals.

Spectrum analyzers are widely used in applications where RF testing is needed: in development, verification and validation testing; production; base service and repair, and increasingly in field installation and service.

In these applications, RF spectrum analyzers are able to provide an effective insight into the RF performance of a circuit, module or system.

As the name spectrum analyzer indicates, this piece of RF test equipment provides information about the spectrum of a signal. As signals on different frequencies can be seen, it enables spurious signals as well as the spectrum of today's complex waveforms to be displayed ensure that thy fall within the required limits on different frequencies.

Although spectrum analysers have traditionally been items of stand alone bench test equipment, rack mount PXI or similar analysers are available along with those that interface to computers, often via USB. Even the bench top test instruments have the capability to link to computers so that they can be controlled by them and also their results analysed in greater detail.



What is a spectrum analyser?

The most commonly used test instrument that displays waveforms is the oscilloscope. This test instrument displays signals in what is termed the time domain, i.e. amplitude against time.

Concept of an oscilloscope screen
Basic oscilloscope screen

Whilst this is vey useful, when testing radio frequency circuits and systems in particular, it is useful to be able to see signals in the frequency domain, i.e. signal amplitudes that appear at different frequencies.

By looking at the amplitudes of signals at different frequencies it is possible to measure the amplitudes of these signals, find what signals are present and the like.

Spectrum analyzer displays signals in the frequency domain
Spectrum analyzer displays signals in the frequency domain

In this way it is possible to measure the frequencies of signals, and also check their levels. With many modern day signals occupying wide bandwidths, it is possible to measure the signal bandwidths.

In this way, the spectrum analyzer is a particularly important item of test equipment for anyone undertaking the test and measurement of circuits and systems involving radio frequency or RF signals. In addition to this, spectrum analyzers may also be used for a variety of other applications including audio analysis and the like.

Typical spectrum analyzer showing RF signal spectrum
Typical spectrum analyzer showing RF signal spectrum

Spectrum analysers normally use a linear scale for the frequency on the horizontal or x-axis, but they normally use a logarithmic scale for the amplitude on the vertical or y-axis. By using a logarithmic or decibel scale for the amplitude scale, it is possible to see signals with large differences in amplitude.

Signals being viewed on a spectrum analyzer may differ by 60dB, 70 dB or more. Using a logarithmic scale is the only way to see these signals on the same screen. For some applications it may be necessary to use a linear amplitude scale, and often there is a switch to accomplish this.

Uses for Spectrum analyzers

Spectrum analyzers are normally complicated pieces of equipment which take a little while to get used to using. However after a little familiarisation, they can become very powerful tools for testing and RF equipment. The spectrum analyzer can be used for a number of tasks:

  • Looking at the frequency spectrum of a signal to see items like the following:
    • The overall spectrum of a modulated signal to see whether it is wide enough or too narrow, etc. If it is too wide then it could cause interference to users in adjacent channels.
    • To investigate whether any spurious or unwanted signals are present. These signals could cause interference to users on other frequencies is signals are transmitted.
    • To find out whether a signal is on the right frequency, and not in another band for example.
  • To investigate general problems with a signal. Often it can just help looking at a signal to see what a problem is. With RF signals a spectrum analyzer can prove to be the eyes for the person investigating the problem.
  • Sometimes spectrum analyzers can be used to measure power, although power meters may be more applicable in certain circumstances.
  • Sometimes spectrum analyzers can be used to measure frequency, although frequency counters may be more applicable in some circumstances.
  • Spectrum analysers can also be used to measure the phase noise on a signal. This can be achieved provided that the pose noise on the spectrum analyzer local oscillator is typically 10dB better than that of the oscillator under test.
  • Another application for these test instruments is that of measuring the noise figure of an item. Although the test method does involve a number of stages, it can be undertaken relatively easily.
  • Spectrum analysers are often used when undertaking EMI & EMI (electromagnetic interference and electromagnetic compatibility) measurements. The analyzer can be used to locate the frequency and nature of the signal that may be causing an issue.

Typical RF spectrum analyzer

Although the RF spectrum analyzer can be used for many radio frequency tests, the table below gives a summary of the different types of test instruments used for RF testing and their typical applications.


Where to use RF Test Instruments
 
Test Instrument Type Frequency measurement Intensity / amplitude measurement Application
Power meter N Y Use for accurate total power measurements
Frequency counter Y N Used to provide very accurate measurements of the dominant frequency within a signal
RF Spectrum analyser Y Y Used primarily to display the spectrum of a radio frequency signal. Can also be used to make power and frequency measurements, although not as accurately as dedicated instruments
RF network analyser Y Y Used to measure the properties of RF devices

Spectrum analyzer key topics

There are several key topics that are associated with RF spectrum analyzers and their use.

  • Spectrum analyzer types:   There are several different types of spectrum analyzer that can be bought and used. Each type has its own characteristics: performance and cost can be balanced to give the best option for any application.

    Older types are normally based around the superheterodyne principle, sweeping the receiver across a band of frequencies and noting the output. More modern spectrum analysers use fast Fourier transforms, FFTs, converting the signal from an analogue to a digital format and then using Fourier analysis to monitor the sign. There are also real time spectrum analysers - these use time overlapping samples to ensure that no transients are lost. Although more complicated internally, they ensure that all signals can be seen.

    There are also options like USB or PC based spectrum analyzers, that use the display, keyboard, etc of the PC, often connected via a USB interface to reduce the cost whilst often maintaining the performance levels.

  • Spectrum analyzer specifications:   When choosing an RF spectrum analyzer it is necessary to understand the specifications and what they mean. There are many different specifications associated with the performance of a spectrum analyzer, and it is necessary to understand what they mean and how they reflect into the actual performance of the test instrument.

    Some of the key specifications include the coverage - top and bottom frequencies, the oscillator phase noise, accuracy of both the frequency and amplitude measurements, and a variety of other specifications.
    Read more about . . . . Spectrum analyzer specifications.

  • How to use a spectrum analyzer:   A spectrum analyzer is a complicated type of test instrument. This summary with a video gives a really helpful guide on their use.

    In order to get the best from any spectrum analyzer, it is necessary to understand the operation of the various controls. For anyone using this type of test equipment fort he first time, it can take a little while to get to grips with the different controls, but soon they become second nature.
    Read more about . . . . How to use a spectrum analyzer.



The RF spectrum analyzer is an essential item of test equipment for any radio frequency engineer. Although spectrum analyzers are normally expensive instruments to buy, they can occasionally be picked up on the second hand market and this would make them accessible to enthusiastic RF experimenters and radio hams.

In addition to this, some USB spectrum analysers offer very high levels of value as they use elements of the PC for display, and entry of the controls. Typically most of the signal analysis is undertaken by an FPGA in the USB spectrum analyzer itself and this means that high levels of functionality and performance can be obtained at a much more reasonable cost.

The spectrum analyzer is an essential item of test equipment used in RF test and development laboratories where they are used extensively, but they are not normally found outside these environments, although audio analysers are used for some specific tests.



More Test Topics:
Analogue Multimeter     Data network analyzer     Digital Multimeter     Frequency counter     Oscilloscope     Signal generators     Spectrum analyzer     LCR meter / bridge     Dip meter, GDO     Logic analyzer     Power meter (RF & microwave)     RF signal generator     Logic probe     Time domain reflectometer, TDR     Vector network analyzer     LabVIEW     PXI     GPIB / IEEE 488     Boundary scan / JTAG    
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